1. Field of the Invention
This invention relates in general to turbine engines.
More particularly, the invention relates to a turbine engine having an improved recuperator which utilizes the high temperature gases exhausted from the turbine to preheat compressed air from the compressor prior to its entry into the combustor.
In a further and more specific aspect, the invention relates to a cogeneration system wherein the waste heat from a recuperated gas turbine engine is exhausted into a boiler or other energy recovery system.
2. Prior Art
Various cycles for converting energy from one form, such as heat, to another form, such as work, are known. Most energy cycles are based on the Carnot cycle composed of the following four reversible processes:
1) A reversible isothermal heat addition; PA1 2) A reversible adiabatic process in which work is done by the system; PA1 3) A reversible isothermal heat rejection; and PA1 4) A reversible adiabatic process in which work is done on the system.
For given temperature limits, the thermal efficiency of a Carnot cycle is the maximum obtainable. Actual energy cycles do not match the efficiency of a Carnot cycle, since the processes involved are not truly reversible and isothermal or adiabatic. However, many cycles have been devised which attempt to approximate the performance of the Carnot cycle. One such cycle is the Brayton cycle, in which a gas turbine is employed.
A simple gas turbine engine or power plant comprises a combustion chamber having inlets for receiving compressed air and fuel, a compressor for compressing the air prior to its entry into the combustion chamber, and a turbine for extracting energy from the hot gases exhausted from the combustor. A portion of the energy extracted by the turbine is used to rotate a drive shaft coupled to the compressor.
Numerous techniques are known for increasing the thermal efficiency, and thereby decreasing the net fuel consumption, of such an engine One common technique is to direct the hot exhaust gases from the turbine through a heat exchanger, known as a recuperator, which heats the comparatively cold air from the compressor prior to its entry into the combustor. As a result, less fuel is required in the combustor for producing a given turbine inlet temperature.
Prior art recuperators have taken a multitude of different configurations. One common type of recuperator is the tubular type, which comprises a plurality of parallel tubes oriented parallel to the engine centerline in an annular matrix with an inlet manifold at one end and an outlet manifold at the other end. Another common type of recuperator comprises a plurality of plates of relatively thin material, so formed and stacked as to provide heat transfer through the plates to and from a series of alternate flow passages formed between the stacked, alternate plates.
Both the tubular type and the stacked plate type of recuperators suffer from a number of shortcomings which reduce their overall thermal efficiency and/or make them impractical for many applications. For instance, the tubular type recuperators utilize a large amount of external ducting, and require a large amount of space in an engine. Thus, they are unsuitable for use in environments such as automotive and jet engines, where compact size and minimal weight are essential. Stacked-plate type recuperators require a large amount of welding and brazing, which means that all the components must be constructed of compatible materials. Thus, even those components which only come into contact with the relatively low temperature (approximately 350.degree. F.) air from the compressor must be constructed from the same high grade alloys as those components which come into contact with the high temperature (approximately 1400.degree. F.) turbine exhaust. This adds unnecessarily to the cost of manufacturing the engine.
Another problem confronting the designers of prior art recuperators has been the high amount of thermal stress due to the large temperature gradients in the different components of the recuperator, and the thermal expansions and distortions which result. Still another problem has been the lack of adequate sealing between adjacent flow passages of the recuperator, resulting in leakage of the high pressure air from the compressor into the low pressure side of the recuperator. Thus the overall pressure ratio, and as a result, the efficiency, of the system is reduced.
Another shortcoming of prior art recuperated engines has been that the recuperator usually encompasses only a small part of the engine. Thus, only a fraction of the waste heat generated by the turbine and combustor actually passes through the heat exchanger. The rest of the heat is lost through radiation.
Another factor affecting the efficiency of gas turbine recuperators is the amount of turbulence within the fluid flow passages. It is generally well known that heat transfer is most efficient when the flow is in the turbulent regime. Commonly, turbulence is induced by inserting strips or rods of twisted metal, known as turbulators, into the flow passages of a recuperator. However, this has only been possible with flow passages of relatively simple construction, such as in straight tube-type recuperators. Other types of recuperators having convoluted or very small-diameter flow passages have not been suitable for the inclusion of turbulators.
In addition to recuperation, other techniques are known for increasing the net efficiency of a gas turbine engine. One technique is cogeneration, in which the waste heat from the turbine is exhausted into an energy recovery system such as a boiler, where it is used to produce steam or other useful energy. A problem with most cogeneration systems, however, is that the energy output varies according to the load. Therefore, as the load decreases, the temperature of the turbine exhaust decreases as well. This is undesirable, since most boilers are designed for constant heat input.
Other ways of increasing the efficiency of an engine include intercooling and reheating. In intercooling, the incoming air is compressed in stages before entering the combustor. Between stages, the air passes through a heat exchanger, known as an intercooler, where the temperature of the air is lowered. In reheating, a second combustor is added for raising the temperature of the gases to a maximum level. Both of these techniques increase the energy output of the engine, since the energy output is proportional to the difference between the lowest and highest temperatures in the system. However, even in these types of systems, a certain amount of energy is wasted, since no attempts have been made to utilize the heat drawn from the compressed air in the intercooler.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to improve the thermal efficiency of a turbine engine
Another object of the invention is to provide a gas turbine engine with a primary surface counterflow recuperative heat exchanger.
And another object of the invention is the provision of a recuperative heat exchanger which entirely surrounds the components of a gas turbine engine.
Still another object of the invention is to provide a recuperative heat exchanger with an improved configuration in which all waste heat is radiated from the hottest point in the engine to the coolest point, to ensure maximum thermal efficiency.
Yet another object of the invention is the provision of a gas turbine recuperator requiring minimal brazing and welding so that multiple alloys can be used.
And yet another object of the invention is to provide a gas turbine recuperator which eliminates the problem of leakage between high and low pressure flow passages.
Yet still another object of the invention is the provision of a gas turbine recuperator which is suitable for the inclusion of turbulators.
And a further object of the instant invention is to minimize the amount of external ducting in a recuperated gas turbine engine.
Yet a further object of the invention is the provision of a cogeneration system wherein the waste heat from a recuperate gas turbine engine is exhausted at a constant discharge temperature into a boiler or other energy recovery device.
And yet a further object of the invention is to provide an intercooled, recuperated and reheated gas turbine engine wherein the waste heat from the intercooler is recovered for useful purposes.
And still a further object of the invention is the provision of a recuperated gas turbine engine and cogeneration system according to the foregoing which can be readily and economically manufactured of conventional materials and in accordance with standard techniques.